1. Field of the Invention
The present invention relates to a device and a method for predistorting signals to be transmitted over a nonlinear transmission path in such a way that the influence of interference segments on the useful part of the signal transmitted over the nonlinear transmission path can be optimized. The present invention may preferably be employed in transmitters for digital broadcasting, which are supplied with signals composed of a number of single carriers and which result in a non-constant envelope of a high-frequency carrier signal. It is particularly in the case of such signals that the nonlinearity of the power amplifier of an output stage of the transmitter gives rise to undesirable frequencies in the signal at the output of the power amplifier which disturb neighbouring frequency channels.
If modulation methods which lead to a non-constant envelope for the high-frequency carrier signal are used in e.g. a wireless communication system, all the signal processing components after the modulator must possess sufficient linearity. This requirement is particularly difficult to fulfil in the case of power amplifier stages designed to operate with high efficiency.
When pulse amplitude modulation methods are employed, the spectral efficiency is degraded by the nonlinearity of the amplifiers. The reason for this is to be found in the nonlinear amplitude output characteristics of an amplifier, which leads to an AM/AM conversion. Furthermore, the drift in the phase of an output signal of an amplifier compared with the phase of an input signal generates intermodulation components, which is also referred to as AM/PM conversion. The AM/AM and AM/PM conversions must be eliminated by suitable linearization methods. If this is not done, the spectral efficiency of the modulation method employed and the signal/noise ratio will be degraded. In digital transmission systems this can result in a considerable increase in the transmission bit error rate.
There thus exists the need for a suitable linearization method based on predistortion. This could be employed in all cases where in general a nonlinear transmission path is to be linearized. The linearization of a nonlinear transmission path should also permit the frequency range of the input signal into the nonlinear transmission path which is to be equalized to differ from the frequency range of the output signal from the nonlinear transmission path. The linearization must not be restricted purely to amplifier stages, therefore, but it must also be possible to perform frequency conversions within the nonlinear transmission path.
Many methods for linearizing high-frequency output stages are already known in this field of technology. The best known methods for linearizing high-frequency output stages are listed below.
2. Description of Prior Art
In digital predistortion of a signal to be transmitted, the digitally represented values of the signal are multiplied by suitably chosen coefficients. The predistortion thus takes place together with the digital generation of the control signal of the modulator.
Another known method is analog predistortion. This employs nonlinear components, e.g. Schottky diodes, to synthesize an equalization characteristic which is the complement of the amplifier distortion characteristic.
The “Cartesian loop” represents an analog negative feedback of the high-frequency output stage which is performed in the baseband.
Forward coupling (also called “feedforward”) represents a disturbance-variable insertion in the sense of control engineering, a corresponding correction voltage being added to the output signal of the output stage so as to compensate for the distortion of the output stage.
In WO 93/18581 a “Cartesian loop” is described whose parameters are adjusted according to various system parameters which reflect the current operating state of the system. Here a radio transmitter comprises a power amplifier, a linearizer and a feedback device for feeding a signal from an output of the power amplifier back to the linearizer so as to ensure the linearity of the output signal. The linearizer operates in the baseband and the IQ signals are controlled by a linear control which is associated with a direct-access table which stores predetermined loop linearization parameters. After the IQ signals have been suitably processed by the linearizer, these processed signals are up-converted by an up-converter and are amplified by the power amplifier. The feedback device takes an output signal of the power amplifier, down-converts it by means of a down-converter and feeds the down-converted signal into the linearizer. The linearization does not therefore take place in the high-frequency range but in the baseband range, since use is made of the IQ signals. Furthermore, the circuit described provides a permanent negative feedback of the high-frequency output stage in the sense of a Cartesian loop.
GB 2240893 A discloses a circuit for linearizing the amplitude response and the phase response of an amplifier. An envelope detector circuit detects the envelope of an input signal to be transmitted and the output signal of the envelope detector circuit is fed into a nonlinear control circuit and into a phase shifter control circuit. The phase shifter control circuit controls a phase shifter, which precedes the power amplifier, so as to predistort the high-frequency signal phasewise. The nonlinear control circuit supplies an input signal to a variable-voltage dc—dc converter, which suitably adjusts the bias voltage parameters, i.e. the operating point of the power amplifier, so as to compensate for the distortion of the nonlinear amplifier. The amplitude error of the amplifier is thus compensated for by adjusting its operating point, which has the disadvantage that the operating point parameters of the amplifier have to be changed continually, which can make it very difficult to match the amplifier to a load since changing the operating point normally automatically entails a different (complex) transformation relationship for the output resistance.
U.S. Pat. No. 5,023,937 describes an analog predistortion circuit for a power amplifier which is operated in the nonlinear range. This predistortion operates by means of a negative feedback loop in which, in contrast to the Cartesian loop, the magnitude and phase of the output signal and not the IQ components are controlled. An envelope detector detects the amplitude of the signal to be amplified and this amplitude, via feedback, is continuously compared with the envelope of the output signal of the power amplifier. The result of the comparison is applied to a variable attenuator which suitably attenuates the input signal to the power amplifier so as to generate an output signal which is as nearly linear as possible. The phase predistortion is performed by means of a phase-locked loop which has the signal to be amplified as its input signal. Part of the output signal of the amplifier is also fed, via a converter, a local oscillator and a phase shifter circuit, into the phase-locked loop, which supplies a local oscillator signal for a converter which precedes the power amplifier and predistorts the phase of the signal to be amplified. The operation of this circuit is completely analog and is based on an essentially continuous feedback provided the phase-locked loop is locked.
U.S. Pat. No. 4,465,980 also describes an analog predistortion circuit. A detector detects the envelope of a signal to be amplified and applies this signal to a field-effect transistor having two gate terminals (dual gate FET). The signal to be amplified is applied to the other gate of the dual gate FET. By suitably controlling the operating point of this dual gate FET the HF input signal is suitably predistorted so as to compensate for the nonlinear amplification of a power amplifier which is connected to the drain terminal of the dual gate FET via a matching network.
DE 3312030 A1 discloses an amplifier with predistortion compensation which employs predistortion components generated by a power amplification element similar to the amplification element so as to achieve an effective linearization which substantially reduces all the intermodulation predistortion products. Furthermore, an additional feedback circuit can also be provided to reduce nonlinearities still further.
EP 312261 A2 discloses a linearization correction circuit which operates in an intermediate frequency range so as to introduce suitable predistortion into an amplitude envelope to compensate for the nonlinearity of the power amplifier stages. An array of parallel current sources, each of which can be adjusted via a corresponding amplitude band in reaction to a predistortion, injects a current which is sufficient to introduce a suitable difference voltage at the output. With this circuit there is obviously no phase predistortion.
EP 0 658 975 A1 relates to a baseband predistortion system for the adaptive linearization of power amplifiers and to a radio transmitter which uses the predistortion system. Here two error tables, one for the amplitude and one for the phase, are actualized and the contents thereof are used to correct the baseband sampled values. The contents of the tables are obtained by accumulating a suitably weighted difference between sampled values, which are fed into the predistortion unit, and a demodulated feedback value. As has already been described for WO93/18581, a predistortion is thus performed not in the high-frequency range but digitally in the baseband. Access to the digital signal conditioning in the baseband must be provided.
A digital predistortion as described in EP 0 658 975 A1 and in WO93/18581 involves access to the modulation signal before it is converted from a digital form to an analog voltage value in order to be able to perform the necessary digital calculations for correcting the carrier amplitude and the carrier phase. In many cases such access is not available since linearization can only be performed within the closed system of the power output stage.
In the case of analog predistortion of the high-frequency signal there is the problem of having to synthesize a suitable characteristic from nonlinear components which are subject to individual differences, temperature drift, ageing, etc. Ageing of the components may result in increased nonlinearity.
The “Cartesian loop”, i.e. a high-frequency negative feedback, reacts very sensitively to parameter fluctuations. As a consequence of the high amplification there is also a strong tendency for the whole arrangement to oscillate if the feedback parameters are not determined precisely. The noise behaviour of the output stage is also seriously degraded by the negative feedback since this introduces uncorrelated noise into the amplifier stage.
The feedforward method requires a precise determination of the signal propagation times of the output stage. The linearization demands made on the correction signal are high, which means that technically advanced, costly power amplifiers must be used in a circuit which performs linearization according to the feedforward method.
A method for the additive correction of an OFDM signal is also known from the article “Reduktion von Nachbarkanal-störungen in OFDM-Funkubertragungssystemen” by Thomas May, Hermann Rohling, TU Braunschweig, Schleinitzstrasse 22, 38092 Braunschweig. In this method the OFDM baseband signal is corrected by means of a suitable auxiliary function to counter-act nonlinearities caused by a subsequent amplifier. The auxiliary function is a suitably adapted si function which ensures that there is no adjacent channel interference since the interference power is concentrated on the OFDM bandwidth so that there is practically no out-of-band radiation.
Also known from DE 19631388 A1 is a system for predistortion for a nonlinear transmission path in the high-frequency range where the envelope of a signal to be transmitted over a nonlinear transmission path is detected, whereupon quantized envelope values are formed. Complex predistortion coefficients, which depend on the quantized envelope values and on a transfer function of the nonlinear transmission path registered beforehand, are stored in a table unit. In addition an evaluator is provided, e.g. an IQ modulator, via which the signal to be transmitted is modulated with the complex predistortion coefficients prior to transmission of the signal over the nonlinear transmission path in such a way that the predistortion caused by the nonlinear transmission path is substantially compensated for as regards magnitude and phase. It has turned out, however, that complete compensation is scarcely achievable with this system and furthermore interference segments still remain in the immediate neighbourhood of the useful channel of the transmission signal. Much is therefore required of a bandpass filter at the far end of the nonlinear transmission path.
In most of the known methods described above for linearizing high-frequency output stages the aim is to reduce the effect of the nonlinearity of the amplifier by providing it with a suitably predistorted input signal. In this way a nonlinear behaviour of the combination predistorter/amplifier within the permissible dynamic range can be achieved. The known predistiorters remember nothing. As soon as the amplifier is so strongly driven for a short time that it acts as a limiter, the predistortion becomes ineffective. The feedforward methods which have also been described represent a disturbance-variable insertion in the sense of control engineering, as already mentioned, a disadvantage being that an amplifier with a performce almost equal to that of the amplifier to be linearized is needed to generate the correction signal.
For input signals with a high crest factor, which occur e.g. in multicarrier methods, the known methods are ineffective since the power amplifier momentarily enters the limiting region.